Integrated circuits are common in electronic products. Electronic products often are comprised of integrated circuits interfaced to each other via a data bus or other data paths. Interface specifications for various digital logic families delineate voltage and current levels required for digital signals to be transferred between two or more integrated circuits. Interface specifications are utilized by integrated circuits through the use of output buffer circuits to drive a logical low or logical high signal across a data path. In addition, output buffer circuits are a way of interfacing different digital logic families of integrated circuits.
Typically, output buffer circuits often use an external voltage level, Vccq, as a source of the logic high level. Depending on the design, Vccq can range from 1.6 to 3.3 volts. Output buffer circuits generally use a system ground (GNDQ) as a sink for a logic low output. Output buffer circuits generally use two complementary transistor devices. The first device is a p-channel pull-up metal-oxide semiconductor (MOS) transistor, whose source is connected to Vccq, and whose drain is connected to the output terminal. The second device is an n-channel pull-down MOS transistor, whose drain is connected to the output terminal, and whose source is connected to ground.
The output buffer of an integrated circuit must be sized large enough to provide sufficient sinking and sourcing to a load and to transmit a signal within a short time. This requirement can demand a high rate of change in the current used in the integrated circuit, and can cause significant noise due to power line voltage drops and/or ground voltage line bumps. This noise can upset nominal operation of the circuits that share a power/ground bus with the output buffers. In modern applications, where integrated circuits are assembled in very small printed circuit boards (PCBs) with specialized routing, the load that an integrated circuit must drive can be much smaller than the load used as reference to design the output driver size.
One example of an application of an output buffer is in a memory system. A memory system is commonly used in products such as digital cameras, personal digital assistants, cellular telephones, video game systems and the like. A typical memory system is used to store commands or data that will be used in conjunction with a microprocessor. With the development of faster and faster microprocessors, memory systems must also keep pace. Fast transition times are a factor in the design of increasing circuit speed. This is particularly true with respect to memory systems.
In some applications, a particular integrated circuit will be used in a variety of applications having a very wide range of output loads. For Flash memory circuits, the capacitive load is typically 30 picoFarads (pF). Most applications have loads much smaller than that. With a standard capacitive load, it is easy to size an output buffer during manufacture for a known standard capacitive load. However, since the variety of applications for which typical integrated circuits are used, a standard output buffer size is not sufficient. For example, with a strong output buffer and small load, overshoot and undershoot on output signals occurs. With a weak buffer, the buffer is insufficient to work with an application having a larger load.
Currently, many IC applications are quite sensitive to radio frequency (RF) interference, and the noise generated by a buffer harms the RF performance of the circuit board. This RF interference increases when the switching time of the loads is too fast, which occurs when the load is small.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for adjusting the strength of an output buffer according to the application for which it is to be used.